Plastic materials have been recently increasingly used as various optical materials, in particular, as lens for spectacles, because of light weight, high toughness and good dyeing ability. The optical materials, especially lens for spectacles, are required to exhibit good properties such as low specific gravity, high transparency and low yellowness as well as optical properties such as high refractive index and large Abbe number. The high refractive index reduces the thickness of lens and the high Abbe number reduces the chromatic aberration of lens. The present inventors found novel episulfide compounds capable of providing optical materials having a small thickness and exhibiting a low chromatic aberration attributable to its refractive index of 1.7 or higher and Abbe number of 35 or larger, and have filed patent applications (Japanese Patent Application Laid-Open Nos. 9-71580, 9-110979 and 9-255781). However, the episulfide compounds cause precipitation or formation of polymers and yellowing within several days depending on storage conditions, especially, cause severe precipitation when solidified. The polymerization of such episulfide compounds results in optical materials having deteriorated quality due to cloudiness and yellowing. Therefore, to produce optical materials stably from episulfide compounds, it has been demanded to develop a method for stably storing the episulfide compounds for a long period of time.